Makehuman rigged model appear all purple in weight paint mode
I've seen this happen on a few models I've come across, but its an easy fix.

To see the weight paint, you need to unhide and select the deformation bones which are tucked away in Object Data under Skeleton - Layers. Ignore the Protected Layers bit.



I don't have a MakeHuman model handy, but this pic shows where the deformation bones are for both Rigify and Pitchipoy, labeled with a DEF- prefix. Keep in mind that you may no be able to move DEF bones directly, so keep the both this layer and the bone control layer active (holding Shift and click on the layer).

Both these rigs also have hide/unhide buttons for all the different parts at the Properties Panel, not sure if yours will also have it.It works the same as above only that the buttons have names for the part they are assigned to.